What is a Mortise Lock?

Written by
Doityourself Staff

on Apr 07, 2010

A mortise lock is a good, secure way to lock a door. It’s typically used on older doors and more generally found in older construction. The mortise lock has been replaced in many construction plans by the modern bored cylindrical lock but has been enjoying a comeback of sorts. Those wanting to install a mortise lock on their homes often find that it’s a complex project beyond the skills of the average homeowner and also requires an intermediate knowledge of woodwork.

How Does A Mortise Lock Work?

A mortise lock needs a mortise, or a pocket, in the door frame to operate correctly. The bolt of the lock fits into the mortise. Because it uses the strength of the door frame, a mortise lock is far more secure than other locks that attach to the outside of a door frame.

There are four parts to a mortise lock other than the mortise itself. The main one is the lock body. This is the part that is fitted within the door. You won’t usually see it but it controls the opening and closing of the bolt when you turn the key. The lock trim varies widely according to the taste of the buyer and can range from lever pulls or handle sets to doorknobs.

The strike plate surrounds the mortise in the door frame and keeps it even stronger, thus preventing any tearing of the wood. Finally, there’s the keyed cylinder. This operates the lock. It should be noted, however, that in many countries the locking mechanism itself isn’t a keyed cylinder but a lever mechanism. However, keyed cylinders have been the mortise lock mechanism of choice in the US for many years.

Functionality

The fact that it’s necessary to remove more wood from the door and the frame means that a mortise door actually weakens the structure of a wooden door. However, the lock itself is stronger and the weaker door structure is really irrelevant. A mortise lock is more versatile than a bored cylindrical lock since it can take a broad range of cylinders by different manufacturers. This way, the lock can easily fit in with others in the same building.

Fitting A Mortise Lock

A mortise lock is much harder to fit than a bored cylindrical lock since it requires precise removal of the wood from the door to cater for the fitting of the lock. Even the fitting of the trim to a mortise lock requires experience, thus putting the whole project outside the ability range of most homeowners. Indeed, specialists have mortising jugs to cut the pocket on the door frame so it’s completely precise. It can take a long time to remove the wood with a chisel so use of a jug is recommended.

Security

A properly fitted mortise lock is very secure. The bolt can be forced with a plastic card if the fit isn’t snug but it’s extremely difficult to pick a mortise lock. With mortise locks on inner doors, single or double levered locks are used. The deadbolt on the mortise lock is heavy and is opened and closed by the teeth on the key connecting with levers in the lock mechanism. With a large number of possible combinations, picking the lock can be an almost impossible task.